Step 7/15 - Fermentation: The Science


Once you have a wash with an appropriate Brix level and a yeast strain, we are ready for the next step. Yeast is typically pitched by tipping 20kg bags of living culture in by hand, or by pumping liquid yeast in through pipes. Some distilleries use propagation tanks with a small amount of wash and optimal conditions for yeast growth before adding it to the rest of the sugary liquid.


In order to create the alcohol and flavors we want, yeast needs sugar and nitrogen. Sugar cane is low in nitrogen, so this is often added. Sometimes the pH level is adjusted with supplements, since yeast thrives in acidic environments, and the ideal temperature for yeast is around 15-30°C. Under these optimal conditions, the yeast gets to work metabolizing sugar into ethanol. Some by-products are methanol, fusel alcohols, fatty acids, sulfur, and esters. Heat & CO2 are waste products during this stage. Because heat is created, distilleries in warm climates have to cool down the wash to ensure that it doesn’t get too warm and kill the yeast.


There’s an incredible amount of variables to consider during this stage. Mineral content and pH level of the wash, the length and temperature of fermentation as well as the type of yeast are key factors. Esters play a significant role in the aromatic make-up of rum. They are formed when fatty acids react with different alcohols. The longer the fermentation, the more acidic the wash becomes, and the more esters are created. Some will have fruity & floral aromas, while others more earthy and oily aromas.


Jamaica is famous for heavier, flavourful rums due to longer ferments, but also employ some other tricks that we will tackle next...Stay tuned!